[Analogpoint]

Just off the top of my head, you could probably hack this...

1. create a forum for the profile comments
2. hack the forumdisplay template to not include you profile comments forum (it probably can't be a hidden forum, as then the threads wouldn't be visible to your users)
3. use a hook to create a new thread in this forum whenever a new user registers (to make this retro active, you'd have to run an SQL query to create a new thread for each of your current users)
4. hack the profile display template to include a display of the user's thread below their profile. (depending on how you access the threads here, you might be able to skip #2 and use a hidden forum [don't check permissions when displaying/posting to these threads])

That's not really polished, but can probably be done without too many problems...